1. Field of the Invention
This invention relates to molds used to cast plastic progressive eyeglass lenses, and to methods of making such molds. This invention also relates to lenses cast using these molds and methods.
2. Description of Related Art
Progressive eyeglass lenses typically include a front face, a back face, a near vision correction zone, a far vision correction zone, and a vision transition zone located between the near vision correction zone and the far vision correction zone. The front faces of these lenses are typically substantially convex, and back face of these lenses are typically substantially concave. These lenses are called xe2x80x9cprogressivexe2x80x9d lenses because their power changes gradually and progressively (i.e., from the power of the near vision correction zone to the power of the far vision correction zone) in the vision transition zone. These lenses are generally preferred compared to ordinary or blended bifocal lenses because wearers of progressive lenses can focus on objects at near distances (by looking through the near vision correction zone), far distances (by looking through the far vision correction zone) and distances between near and far distances (by looking through different parts of the vision transition zone). Bifocal lenses only allow wearers to focus on objects at near and far distances since bifocal lenses only have a near vision correction zone and a far vision correction zone.
Molds used to cast progressive eyeglass lenses are hereinafter referred to as xe2x80x9cprogressive molds.xe2x80x9d These molds typically include a back face which is adapted to cast: (1) a near vision correction zone in the lower portion of a lens when worn, (2) a far vision correction zone in the upper portion of a lens when worn, and (3) a vision transition zone between the near and the far vision correction zones. Progressive molds are typically used to cure lens forming compositions to form lenses. They are typically used in combination with a second xe2x80x9cbackxe2x80x9d companion mold and a gasket to form a mold cavity. In one embodiment the progressive mold and companion mold seal against opposite sides of the gasket to form the mold cavity. The mold cavity is then filled with a curable liquid composition, and the liquid composition is cured to a solid by exposure of the composition to light or heat.
Plastic eyeglass lenses are typically cured in the form of a lens xe2x80x9cblankxe2x80x9d which is close to, but generally not identical to, the prescription needed by the lens wearer. This blank must be further shaped (e.g. cut, beveled, or ground) to closely fit specific customer prescriptions. This final xe2x80x9cprescription shapingxe2x80x9d may be completed in a retail environment, or a wholesale laboratory environment.
Recently new methods have been developed for preparing lenses by casting lens-forming compositions between glass molds such that the cast lenses fit specific wearer prescriptions. These cast lenses do not require xe2x80x9cprescription shapingxe2x80x9d since they are cast to meet the final prescription needs of the wearer. The lens-forming compositions in these processes may be cured by exposure to ultraviolet light or heat, or by any free radical polymerization process. Ultraviolet light processes are adaptable to produce lenses in time periods of less than one hour. As a result, lenses can be directly cast, without further shaping, to fit customer prescriptions and still provide quick service. Some of such curing techniques are described in the U.S. patents and co-pending applications described below:
co-pending U.S. application entitled xe2x80x9cApparatus and Process For Lens Curing and Coatingxe2x80x9d by Buazza et al., filed the same day as this application, and which is a continuation-in-part of co-pending U.S. application Ser. No. 800,561, filed Dec. 6, 1991, which is a continuation-in-part of co-pending U.S. application Ser. No. 642,614, filed Jan. 17, 1991, which is a continuation-in-part of co-pending U.S. application Ser. No. 425,371, filed Oct. 26, 1989, which is a continuation-in-part of Ser. No. 273,428, filed Nov. 18, 1988, now U.S. Pat. No. 4,879,318, which is a continuation-in-part of Ser. No. 021,913, filed Mar. 4, 1987, now abandoned, which is a continuation-in-part of Ser. No. 823,339, filed Jan. 28, 1986, now U.S. Pat. No. 4,728,469.
These patents and co-pending applications are incorporated by reference. In addition, U.S. Pat. No. 4,919,850 also describes methods and apparatus for making plastic lenses, and this patent is also incorporated by reference.
Progressive molds may be made using a variety of methods well known in the art such as grinding. Grinding progressive molds, however, can be unduly expensive and difficult due to the uneven back face of the mold. An alternate method to make progressive molds is a xe2x80x9cheat slumpingxe2x80x9d process. This process is preferred by some manufacturers for preparing progressive molds because it is relatively less expensive than other processes such as grinding.
When making progressive molds using a heat slumping process, a first or prototype surface may be prepared which is a replica of the front face of the progressive lens desired. This first surface may be concave or convex, and typically comprises a ceramic material. After the first surface has been prepared, mold material is placed over this first surface. Typically this mold material is made of glass and is shaped in the form of a disk. The mold material and the first surface are then heated to a high enough temperature such that the mold material xe2x80x9cslumpsxe2x80x9d or molds around the first surface, thereby forming a progressive mold with a back face substantially similar to the front face of the lens desired. The resulting mold also has a front face, and side surfaces connecting the front face to the back face. The mold is then removed from the first surface.
Whether made by grinding, heat slumping, or other processes, one problem with nearly all existing progressive molds is that they tend to cast lenses that have xe2x80x9cvertical prismxe2x80x9d effects. If uncorrected, vertical prism effects cause wearers of such lenses to see images at different vertical levels than the images really are (i.e., the lenses tend to cause the wearer""s vision to shift slightly in the vertical direction). Wearers of lenses with pronounced vertical prism imbalance tend to experience diplopia (double vision), nausea, and/or suffer headaches. Vertical prism effects are believed to be caused by excessive differential thicknesses between the near and far vision correction zones in progressive lenses. Specifically, the bottom of the lens proximate the near vision zone is thicker than the top of the lens proximate the far vision correction zone. If the amount of thickness difference is excessive, it may cause an unacceptably large amount of (base down) vertical prism effects.
Vertical prism effects exist but are relatively easily corrected in progressive lens blanks. The correction takes place when the lens blanks undergo prescription shaping. The prescription shaping process usually involves applying a shaping instrument (such as a grinder) to the center of the concave surface of the lens. To hold the lens blank in place, the convex surface of the lens blank is attached to a lens xe2x80x9cblock.xe2x80x9d This lens block is attached to the geometric center of the convex surface of the lens blank. Any desired prism effect correction is made by inserting a xe2x80x9cprism ringxe2x80x9d around the block such that it can seat against the lens blank. The prism ring is designed to tilt the lens blank the appropriate amount necessary to increase or decrease vertical prism effects in the lens blank when the lens blank is shaped to prescription. The tilting caused by the prism ring also affects the thickness of the edges of the resulting lens. Sometimes practitioners will vary the vertical prism effects (i.e. by varying the prism ring tilt) in an effort to cosmetically optimize lens edges. This process is well known in the art.
Vertical prism effects are more difficult to treat with lenses that cast to prescription since these lenses do not undergo a prescription shaping process. Thus vertical prism effects remain in these lenses unless other-measures are taken to reduce or eliminate them. These measures are the subject of the invention described herein.
In a general aspect, the present invention includes a system to reduce vertical prism effects in prescription lenses cast using progressive molds. In another aspect, the invention includes a mold for casting prescription progressive lenses with reduced vertical prism effects. The invention also includes progressive lenses made by the system of the invention.
In a more specific aspect, the invention is directed to a progressive mold disk with a front face, and a back face which is concave and tracks the convex front face of a lens desired to be cast with the mold. The mold disk is bounded by a generally cylindrical surface which joins the front and back faces. In addition, the back face has a relatively narrow annular surface or band which extends around its outer edge or periphery. This annular surface or band is relatively flat and extends between the outer boundary of the concave portion of the back face and the outer boundary of the back face itself. The width of the band varies around the periphery of the back face. It is usually widest at the portion of the back face corresponding to the bottom of the near vision correction zone of a lens formed by the mold. It is narrower at the portion of the back face corresponding to the top of the far vision correction zone of a lens formed by the mold. In general, these two extreme portions of the band are both wider than the remaining intermediate portions.
A key feature of the mold component of the invention resides in the formation of the annular surface or band around the edge of the back face. This surface or band is formed by tapering the back face of the mold at a xe2x80x9ccorrectionxe2x80x9d angle such that more of the back face near the bottom of the near vision correction zone is removed versus the back face near the top of the far vision correction zone. The surface or band near the bottom of the near vision correction zone thus tapers or slopes slightly inward and towards the center of the mold. The surface or band near the top of the far vision correction zone tapers or slopes slightly inward and away from the center of the mold. The angle of taper or slope corresponds to the correction angle.
An advantage of the invention is that vertical prism effects can be corrected in cast prescription lenses without modifying the gasket or the molds used to cast the back face of such lenses (i.e. the xe2x80x9cback moldsxe2x80x9d). As a result, several different prescription progressive lenses can be cast using identical gaskets and back molds. In such case, often only the progressive molds themselves need to be varied to make different progressive lenses.